1. Field of the Invention
The present invention generally relates to electrical circuit boards and their fabrication. More particularly, this invention relates to a method for providing a planar surface on a circuit board to allow screen printing a polymer thick-film resistor with improved dimensional and electrical tolerances.
2. Description of the Prior Art
Thick-film resistors are employed in hybrid electronic circuits to provide a wide range of resistor values. Such resistors are formed by printing, such as screen printing, a thick-film resistive paste or ink on a substrate, which may be a printed wiring board (PWB), flexible circuit, or a ceramic or silicon substrate. Thick-film inks used in organic printed wire board construction are typically composed of an electrically-conductive material, various additives used to favorably affect the final electrical properties of the resistor, an organic binder and an organic vehicle. After printing, the thick-film ink is typically heated to dry the ink and convert it into a suitable film that adheres to the substrate. If a polymer thick-film (PTF) ink is used, the organic binder is a polymer matrix material and the heating step serves to remove the organic vehicle and cure the polymer matrix material.
The electrical resistance of a thick-film resistor is dependent on the precision with which the resistor is produced, the stability of the resistor material, and the stability of the resistor terminations. The xe2x80x9cxxe2x80x9d and xe2x80x9czxe2x80x9d dimensions (the width and thickness, respectively, of the resistor) of a rectangular PTF resistor are typically determined by a screen printing process, while the xe2x80x9cyxe2x80x9d dimension (the electrical length of the resistor) is determined by the resistor termination pattern. Conventional screen printing techniques generally employ a template with apertures bearing the positive image of the resistor to be created. The template, referred to as a screening mask, is placed above and in close proximity to the surface of the substrate on which the resistor is to be formed. The mask is then loaded with a PTF resistive ink, and a squeegee blade is drawn across the surface of the mask to press the ink through the apertures and onto the surface of the substrate. Copper terminations are typically formed prior to deposition of the ink by additive plating or electrolytic panel plating with subtractive etching, both of which are capable of achieving a high level of edge definition that enables accurate determination of the electrical length (y) of the resistor.
Compared to many other deposition processes, screen printing is a relatively crude process. As a result, screen-printed PTF resistors are typically limited to dimensions of larger than about one millimeter, with dimensional tolerances generally being larger than about xc2x110% at this lower limit. While the y dimension (electrical length) of a screen-printed PTF resistor can be accurately determined by using appropriate processes to form the terminations, control of the x and z (width and thickness) dimensions of a PTF resistor is fundamentally limited by the relatively coarse mesh of the screen and by ink flow after deposition. Control of resistor dimensions is further complicated by the variability of the surface on which the resistive ink is printed, due in large part to patterned copper interconnects for these resistors having typical thicknesses of about ten to thirty-five micrometers. The interconnect prevents a smooth squeegee action across the surface, resulting in imperfect printing of the screen image and non-uniform deposition of the resistor ink. Consequently, resistance tolerances of less than xc2x120% are difficult to achieve with screen-printed PTF resistors without laser trimming, an operation that is usually cost prohibitive for complex circuits.
From the above, it can be seen that what is needed is a method for forming PTF resistors with more accurate dimensions. Fully additive electroless plating can be used to produce copper interconnect that is substantially coplanar with the dielectric, yielding a planar board surface that enables improved printing precision. However, electroless plating is a very slow and expensive process compared to electrolytic panel plating and subsequent subtractive etching.
According to the present invention, there is provided a method of manufacturing a printed circuit board with a polymer thick-film (PTF) resistor whose dimensions can be defined with improved precision by providing a circuit board construction having a planar surface where the resistor is to be deposited. To achieve the desired board construction, the interconnect for the resistor is electrolytically pattern plated using a permanent photodielectric layer as a plating mask instead of a sacrificial plating resist. The interconnect can be patterned before or after the PTF resistor ink is printed. The x and z dimensions (width and thickness, respectively) of the resistor are determined by the deposition process, while the y dimension (electrical length) is accurately determined by copper terminations.
The method of this invention generally entails forming a first electrically-conductive layer on a dielectric substrate, forming an opening in the first electrically-conductive layer to expose a surface portion of the substrate, and then forming a dielectric layer that covers a part of the exposed surface portion of the substrate and preferably adjacent surface portions of the first electrically-conductive layer, while exposing a surface portion of the first electrically-conductive layer. Using the dielectric layer as a mask, a second electrically-conductive layer is then deposited on the first electrically-conductive layer so that the dielectric layer and the second electrically-conductive layer define a substantially coplanar surface. In a preferred embodiment, portions of the first and second electrically-conductive layers are then removed to define a pair of terminations separated by the dielectric layer, after which a polymer thick-film resistive material is screen printed on the dielectric layer and the terminations to define a polymer thick-film resistor. Alternatively, the polymer thick film resistive material can be screen printed on the dielectric layer and the second electrically-conductive layer prior to the terminations being defined.
From the above, those skilled in the art will appreciate that the method of this invention is able to produce PTF resistors whose thickness can be more accurately controlled as a result of the thick-film resistor ink being deposited on a substantially planar surface region. In the preferred embodiment, though the surface area of the planar surface region is relatively smaller for a given resistor size than that possible with the alternative embodiment, a significant improvement in thickness control is nonetheless achieved because sufficient local planarity is provided between road including the terminations. An additional advantage of the preferred embodiment is that the PTF resistor can be tested immediately after printing.
Other objects and advantages of this invention will b e better appreciated from the following detailed description.